DESCRIPTION (adapted from abstract): Retrotransposons have had an important role in increasing repetition in the genome and have thereby had a major effect on evolutionary diversity. The retrotransposon L1 is a long interspersed nuclear element that pervades mammalian genomes in an enormous number of copies, most of which are truncated, rearranged, or otherwise mutated. In recent years, L1 insertions into genes have been found to cause disease in humans and mice. This application proposes to study the mechanism of L1 retrotransposition using a genetic approach, and to demonstrate a significant evolutionary function for active L1s, namely the ability to transduce DNA sequences to new genomic locations. Besides these studies, they will study site-specific retrotransposition in cultured cells by substitution or addition to L1 of endonuclease or DNA binding domains that have sequence specificity. In addition, they will study retrotransposition in vivo in transgenic mice, and use these mice to determine retrotransposition frequency in germ cell and somatic cell development. Lastly, they plan to initiate development of L1 elements into practical mutagenesis agents in mice.